Add credentials for service account, download the generated service account's public/private key

Recommendation: Your application can complete these tasks either by
using the Google APIs client library for your language, or by directly
interacting with the OAuth 2.0 system using HTTP. However, the
mechanics of server-to-server authentication interactions require
applications to create and cryptographically sign JSON Web Tokens
(JWTs), and it's easy to make serious errors that can have a severe
impact on the security of your application.

For this reason, we strongly encourage you to use libraries, such as
the Google APIs client libraries, that abstract the cryptography away
from your application code.

In your server code you should follow this steps

Create a JSON Web Token (JWT, pronounced "jot"), which includes a header, a claim set, and a signature.